1. Field of the Invention
The present invention relates to a laminar flow control system (LFC) and to a suction panel for use therewith, although such a suction panel could have other uses.
2. Discussion of Prior Art
The specific application of the present invention is in a laminar flow control system (LFC) which could be incorporated into an aircraft wing or other aerodynamic surface where it is desired to control the flow of air over the surface.
There are known LFC systems which comprise a panel 1, in the form of a perforated skin 2, typically made of titanium and mounted on large scale plenum chambers 3a, 3b etc. This is illustrated in FIG. 1. The air flow is on the top surface of the system above the panel 1 as indicated by the arrow Z in FIG. 1. To maintain boundary layer control suction is applied through the perforated skin 2 through a series of large scale plenum chambers 3a, 3b . . . as illustrated. The plenum chambers 3a, 3b . . . are connected together by pipework and suction is applied as indicated by arrow Y in FIG. 1. In general, there will be a pressure gradient along the top surface due to the flow of the air. In FIG. 1 for example at the positions of the holes shown the pressures could be P1 greater than P2 greater than P3 . . .  greater than Px (or reverse). It is necessary to make sure that no flow occurs through holes 4 in the skin 2 and in the plenum chambers 3a, 3b . . . to cause this pressure gradient to equalise (a condition known as outflow). To avoid this the pressure drop xcex94Pp across the perforated skin 2 needs to be greater than Px-P1. xcex94Pp is limited by the maximum pitch and the size of the holes 4 that can be drilled. The pitch is limited by the requirement to have effectively uniform suction and the hole size is limited by the drilling process. In addition, there is a requirement that the suction velocity in a particular hole 4 does not exceed a critical value that would disturb the flow.
These operational and mechanical integrity restrictions mean that there is a maximum size for the dimension D of each plenum chamber 3a, 3b . . . . Hence in general for LFC applications there are large numbers of these plenum chambers 3a, 3b . . . . This introduces some serious problems.
For example the pressure of the plenum chambers reduces the overall stiffness of the suction panel. This leads to the need for a thicker, heavier weight stain to improve the stiffness but at the expense of undescribably increasing the weight of the panel.
In areas where a wall 5 of the plenum chamber 3a, 3b . . . is attached to the perforated skin 2 there is no suction. These areas are relatively large in order to provide the necessary mechanical integrity to the structure. This will destabilise the flow and in the worst case could cause a transition to turbulent flow. The problem would get worse with larger plenum chambers as bigger skin support landings would be required. Therefore a significant fraction of the surface contains regions with no suction. In the panel shown in FIG. 1 the perforated skin 2 is typically 1-2 mm thick and the overall depth or thickness of the panel as a whole, i.e. dimension E, is 2-3 cms. Overlap of the holes 4 by wall 5, typically of the order of 3 to 5 mm causes loss of suction. D typically is 3 to 5 cms.
According to one aspect of the present invention there is provided a laminar flow control system which comprises a perforated skin, a plurality of plenum chambers on which the skin is mounted, means for applying suction to the plenum chambers in order to maintain boundary layer suction to a free surface of the perforated skin and micro-channels formed either in the mounted side of the skin or in a support which carries the skin.
According to another aspect of the present invention, there is provided a suction panel for use in a laminar flow control system including a perforated skin, a plurality of plenum chambers on which the skin is mounted and micro-channels formed either in the mounted side of the skin or in a support for the skin.
According to a further aspect of the present invention, there is provided a method of manufacturing a suction panel as specified in the immediately preceding paragraphs including the step of employing a laser beam to form the micro-channels and the perforations in the skin.
According to yet another aspect of the present invention there is provided, an aircraft flying surface incorporating a laminar flow control system as specified in three paragraphs above.